Bag control mechanisms, which are also known as center mechanisms, are used to position a green tire in a tire press or mold, and to provide for the admission and removal of a fluid curing medium. A fluid curing medium may be either a gas, such as steam or a mixture of steam and inner gas, or may be a liquid such as hot water under elevated pressure. A center mechanism typically comprises a cylindrical casing having a fluid actuated piston and piston rod reciprocal therein, a hub having inlet and outlet passageways for curing medium extending therethrough, and a distribution cap or gland which is placed atop the hub. The center mechanism also includes two sets of clamping rings for clamping the ends of a flexible bag or bladder in place. One set of clamping rings (typically the upper set) is typically supported on the piston rod and reciprocates therewith. The other set of clamping rings (typically the lower set) is typically supported on the hub and is stationary. Representative mechanisms are shown in U.S. Pat. No. 3,976,409 to Athey, and in U.S. Pat. No. 4,338,069 to Singh et al. (see especially FIG. 9).
Curing medium is introduced through the hub inlet passageway into the interior space of the bag or bladder, as is well known in the art. Suitable operating cycles are well known; typically, curing medium is supplied at a slight superatmospheric pressure in order to shape the bag, so that it is in contact with the inside surface of a green tire, after which the curing medium is supplied at full curing temperature pressure. Curing medium is exhausted through the outlet passageway in the hub. Suitable operating cycles are described for example in the aforementioned U.S. Pat. Nos. 3,976,409 and 4,338,069, and so will not be described in further detail here.
Presently known center mechanisms have two problems in operation. First and foremost, there is an appreciable variation in temperature between different points within the tire press. This variation may be appreciable, for example, about 7.degree. F. (or approximately 4.degree. C.) between the hottest and coolest portions of the tire press. Typically the upper portion is hotter; condensate (when steam is used) tends to cool down the lower set of bladder clamp rings. This temperature variation results in uneven cure of the tire, since the cure rate is quite sensitive to temperature. Another problem is that curing medium is inefficiently utilized. Both problems stem from the same cause, which is improper design of the fluid inlet and outlet in the hub. Present designs permit cross flow of curing medium from the inlet to the outlet, so that not all of the curing medium is introduced into the interior space of the bladder, and with the further result that the aforesaid temperature variations may occur.